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Stepped footings / Stemwall Design 5

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medeek

medeek

Structural
Mar 16, 2013
1,104
When it comes to footings combined with stemwalls for most of my residential projects I am usually concerned with bearing loads (ie. sizing the footing for 1,500 psf or less) or if there is unbalanced fill I sometimes have to look at retaining wall designs. Up until now I have not given much thought to how the designer/contractor is actually building steps into their footings and stemwalls. Most of my homes, until recent, have been on level ground but as I've seen more jobs with sometimes very complex stepped footing configurations I've begun to give this some more thought.

Are there any specific details or resources which delve into this subject further, both from a construction standpoint and engineering?

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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Typical residential stepped footing:

URL]


I've noticed I don't usually see a step more than 24" in height, that seems to be a good rule of thumb...

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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At the haunch, I always tie the strip footings together with the bottom bars and thickened concrete section, tapering the bottom of the lower footing up to meet bottom of the upper lower strip footing.

I will allow steps up to 4 feet, although 2 feet is common.

Mike McCann, PE, SE (WA)


 
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Agreed with Mike on tying the 2 levels together. I do limit them to 2 ft steps to keep the tapered portion a manageable size (2 ft long).
 
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For your own sanity, it can be helpful to provide some options. Draw it all once and never get queried on it again. Well, at least not too often.

Capture_nytbjk.png


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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KootK,
Alternative 2 probably does not comply with the IBC. It prohibits slopes on the bottom of footings exceeding 1V:10H.
 
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Thank-you KootK for the details, from these I will develop a standard detail(s) to include in my library.

I do have quite a few designers on their plans that will draw something like this (the dimensions and notes are for clarity and questions below):

STEPPED_FOUNDATION_e7sa8l.jpg


The step is 24", they usually don't even show the footing at the haunch, I'm am wondering how this actually gets constructed in the field and what the appropriate detailing should be.

Per KootK's detail above, overlapping the footing would result in an large quantity of concrete at the haunch, or at least to my untrained eye.

I'm also wondering about the overlap of the stemwall above the footing.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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It's strange that this topic has never really come up before for me but I guess since most of my projects have been built on a a sand bar (Ocean Shores, WA) sloping terrain has never been an issue. I have quite a few examples now of other residential engineering plans and details that were done in my vicinity but I don't see any details with regards to stepped footings even though some of the plans do have those features. Its seems that this is ignored by some engineers but I would rather not leave anything to chance.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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wannabeSE said:
Alternative 2 probably does not comply with the IBC. It prohibits slopes on the bottom of footings exceeding 1V:10H.
Click to expand...

Would you mind sharing the clause so that I can check it out for myself? I'm not beholden to the IBC these days. The practical distinction strikes me as a bit wishful. It's not like you're getting the hard 90 degree angles shown in detail one unless you're dealing with a very stiff clay and an unusually talented excavator. Most stepped footing end up being sloped footings to some degree.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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medeek said:
Per KootK's detail above, overlapping the footing would result in an large quantity of concrete at the haunch, or at least to my untrained eye.
Click to expand...

Don't sweat this. Remember, most of the cost is in the formwork. A localized blob like we're talking about here is economically irrelevant.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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I guess I would tend to agree with KootK on this one. Detail 1 is going to approximate detail 2 in most soils.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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Alt 2 per Kootk is my company's standard detail, with the exception that we go ahead and show the diagonal crack control reinforcing inside the wall, (which Kootk probably puts in his wall details). hey wannabese... Please followup on where the IBC prohibits this. 1V:10H seems extremely conservative to me... i could see argument pushing the code to 1V:3H but not 1:10. Is it possible that the 1:10 reference is not discussing stepdowns but the laying of a continuous footing unlevel?
 
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@darthsoilsguy2

Explain the diagonal crack control reinforcing in the wall. Typically what I see for most residential stemwalls is 24" in height with one #4 bar in the top and some vertical #4 bars at 24" or 48" o/c. I only specify something above and beyond that when there is significant shear loads on the wall (ie. high aspect shear walls or portal frames that induce large bending moments into the stemwall/footing below).

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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The IBC section on steeped footing is §1809.3
IBC said:
1809.3 Stepped footings. The top surface of footings shall be level. The bottom surface of footings shall be permitted to have a slope not exceeding one unit vertical in 10 units horizontal (10-percent slope). Footings shall be stepped where it is necessary to change the elevation of the top surface of the footing or where the surface of the ground slopes more than one unit vertical in 10 units horizontal (10-percent slope).
Click to expand...
 
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Step Details my firm has used since'97 and previous by my Father since the early/mid 60's. We have worked with a lot of foundations on slopes, including walk-out basements.
D-STEP2_ucumz4.jpg
 
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You can't upload things to engineering.com that have spaces in the title. Found that out once upon a time.
 
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BigH said:
How do you embed the photo in the post??
Click to expand...

1) Bring the thing up on screen.
2) Use the windows snipping tool to capture the image and save it someplace.
3) Use the button circled below to select the file for inclusion in your post.

#2 can be skipped but can lead to weird results with respect to display size etc.

Capture_01_lhu3xc.png


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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wannabeSE said:
The IBC section on steeped footing is §1809.3
Click to expand...

Thanks for that wannabeSE. I can empathize with your interpretation but, alas, I've been interpreting it differently.

IBC Statement said:
The top surface of footings shall be level. The bottom surface of footings shall be permitted to have a slope not exceeding one unit vertical in 10 units horizontal (10-percent slope). Footings shall be stepped where it is necessary to change the elevation of the top surface of the footing or where the surface of the ground slopes more than one unit vertical in 10 units horizontal (10-percent slope).
Click to expand...

KootK Interpretation said:
A footing, in the macroscopic sense (pad, long run of wall strip), shall not have it's bottom surface sloped greater than 10H:1V. Where the grade of the underside of the footing must drop at a rate steeper than 10H:1V, that shall be accomplished by way of localized footing steps which may, themselves, have their lower surfaces sloped at an angle greater than 10H:1V as approved by the geotechnical engineer.
Click to expand...

Is my interpretation valid? Dunno. I've been able to get it past code reviewers for good long time. I cling fervently to my previously expressed opinion that it's not realistic to expect those hard 90's in detail one. Neither is it realistic to accomplish a footing "step" by way of incremental slopes not exceeding 10H:1V

I found the blurb below at the journal of light construction website which takes yet another tack. They don't count the portion of the soil in the "wedge" as bearing. I've not accounted for that but I can certainly see the logic in it.

Capture_01_k2z6gu.png


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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